1. Field of the Invention
The present invention relates to a method for manufacturing a very-large-scale-integrated (VLSI) semiconductor device, and more particularly, to a method for manufacturing a capacitor for a VLSI semiconductor device. The present application is based on, and priority is claimed from, Korea Patent Application No. 97-36165, filed Jul. 30, 1997, which is incorporated herein by reference for all purposes.
2. Description of the Related Art
Recently, many efforts to increase capacitance within a limited cell area have been focused on using thin films of high dielectric constant material, e.g., a high dielectric constant material having a perovskite structure. In order to be useful as a high dielectric constant thin layer for a capacitor, a material must meet the following requirements: (1) low leakage current; (2) large dielectric constant; and (3) excellent step coverage.
A method for decreasing the leakage current level of a high dielectric constant thin film while improving the dielectric constant is disclosed by Hwang et al. in a thesis entitled "Deposition of Extremely Thin (Ba, Sr) TiO.sub.3 Thin Films for Ultra-Large-Scale Integrated Dynamic Random Access Memory Application," Appl. Phys. Lett. 67(19), Nov. 6, 1995. According to this thesis, after forming a lower electrode, a (Ba, Sr) TiO.sub.3 (BST) film is deposited at a substrate temperature of 660.degree. C. using a sputtering technique. Subsequently, the BST film is annealed under a nitrogen (N.sub.2) atmosphere at a temperature of 750.degree. C. It has been known that leakage current of a capacitor decreases and dielectric constant is improved when the BST film is annealed under an N.sub.2 atmosphere, since a potential barrier is formed between an electrode and a BST film. However, according to this thesis, since the BST film is deposited by sputtering technique, step coverage of the BST film is lowered to 50% or less.
Compared to the BST film formed by sputtering technique, a BST film formed by a chemical vapor deposition (CVD) method shows improved step coverage. However, as the chemical vapor deposition temperature for forming the BST film is increased, the step coverage of the BST film is reduced. Accordingly, the step coverage of the BST film which can be achieved is limited so long as the BST film is formed at a deposition temperature of 500.degree. C. or more. On the other hand, when the BST film is formed at a temperature less than about 500.degree. C., step coverage is improved while dielectric constant of the BST film is decreased.
A method for manufacturing a capacitor having a BST film with improved step coverage and dielectric constant, is disclosed by T. Kawahara et. al. in a thesis entitled "(Ba, Sr) TiO.sub.3 [BST] Films Prepared by Liquid Source Chemical Vapor Deposition on Ru Electrode," Jpn. J. Appl. Phys. Vol. 34, 1996, pp 5077-5082. According to this method, a lower electrode is formed on a semiconductor substrate. A first BST buffer film is then deposited on the electrode at a substrate temperature of about 420.degree. C. and to a thickness of 50.about.60 .ANG.. Subsequently, the first BST film is annealed at 700.degree. C. under a nitrogen (N.sub.2) atmosphere. Successively, the substrate temperature is set again to 420.degree. C., and a second BST film is deposited on the first BST film. Next, the substrate temperature is raised to 700.degree. C. and the second BST film is annealed and crystallized. An upper electrode is then formed on the crystallized second BST film, thereby completing the capacitor. The capacitor formed in this way has improved electrical properties because the BST film is annealed at high temperature under an N.sub.2 atmosphere, and the BST film step coverage is increased because the BST film is formed at low temperature. According to this method, however, a two-step process must be repeatedly performed, that is, a deposition step and an annealing step is sequentially repeated twice. Thus, the fabrication process is complex. To simplify the fabrication process, the BST film may be annealed at 700.degree. C. under N.sub.2 atmosphere immediately after forming the BST film by a single step. However, in this case, the leakage current of the capacitor is increased and the dielectric constant thereof is lowered.